Evaporative coolers or air conditioners are commonly used in dry areas of the world. One drawback of such systems is that even though they reduce the temperature of the air pumped into a space for human comfort, they simultaneously raise the humidity of the air often to the point where the human perception of greater comfort is significantly reduced. It is well known in the air conditioning industry that human beings feel cooler in air which has a lower temperature and humidity than the ambient air. The feeling of greater comfort is partly due to the fact that drier air promotes natural evaporation of moisture through human skin, thereby creating a perception that the air is cooler.
The present invention is an improved flat plastic plate heat exchanger involving the evaporative cooling process whereby the cooler but more humid air from the evaporative cooling process is used as a medium to absorb heat from the air being introduced into the space for human comfort without raising the humidity of that air. Additionally, the heat exchanger enables the air in the space for human comfort to be recycled through the heat exchanger for additional cooling, unlike a plain evaporative cooling system where the air from the cooler goes only once into the space for human comfort and then must be expelled from that space. The heat exchanger may also be used as a precooler for the intake of a conventional mechanical air conditioner, or a conventional direct evaporative cooler, where the precooler does not raise the humidity of the air going into the intake.
The basic components of an evaporative cooling system are a wetable medium (such as a fibrous pad, a sump to hold water, a pump to discharge water over the medium or pad, and a fan to move air past or through the wetted medium. Incoming air of relatively low humidity can thus be cooled by passing over or through the medium and then delivered to a space for human comfort or for cooling. One of the main advantages of a basic evaporative cooler is very low cost.
A heat exchanger is necessary to avoid raising the humidity in the space for human comfort. The preferred embodiment of the present invention involves indirect evaporative cooling because the evaporative cooling takes place when water in the secondary air stream passages absorbs heat from the primary air stream, and then the water is cooled when it evaporates in the secondary air stream in the invention. The heat from the primary air stream is thus transferred through the work of the heat exchanger to the cooler air in the secondary air stream. The heat exchanger can be operated without water in the secondary air stream passages, so that there is direct heat exchange, through the exchanger plates, between the primary and secondary air streams, but the efficiency is much less. However, most heat exchangers of conventional design are difficult to make and seal so that there is no air or water leakage between the two air streams. While metal can be a desirable material for a heat exchanger, it is costly, heavy, and difficult to fabricate and tightly seal. Plastic is a material with a lower index of heat transfer, yet it is very light, inexpensive, easy to fabricate through thermomoulding, yet it is difficult to seal, and may leak, lowering cooling efficiency because of water or air leakage between the two air streams. In particular, conventional flat plate plastic heat exchangers suffer from leaks due to fatigue cracks and thinning in the plates and leaks due to failure of the plate edge seals. Another disadvantage of conventional plastic plate heat exchangers is that the plates tend to deform or sag thus increasing the size of some air flow chambers and decreasing the size of other airflow chambers. It is an object of the present improvement invention to lower or eliminate leaks in plastic plate heat exchangers. It is another object of the present invention to maintain more uniform spacing between the plates of said exchangers. It is another object of the present invention to facilitate uniform assembly of the stack of plates and to provide more uniform plate edge sealing.
Inventions previously known have used plastic plates with more complex construction (U.S. Pat. No. 4,907,648; Mar. 13, 1990 to Emmerich et al.). More complex metal plates with dimples are disclosed in U.S. Pat. No. 5,487,424; Jan. 30, 1996 to Davison; however, this invention has double layer welded metal plates which are costly to make.
The preferred embodiment of the invention comprises a stack of square or rectangular plastic plates. The plates are formed of thermoplastic material (preferably a PVC/CPVC composite plastic approximately 8 mils thick with resistance to ultraviolet light and heat) formed by vacuum molding in a heated mold. Heat softens the plastic, and the vacuum draws it over the mold. The mold is quick cooled with water to get a true set and eliminate warping. The mold comprises a flat plate in which there are raised flat top dimples regularly spaced across the flat base. The dimples have large radii curves around the flat top. The diameter of the circular flat top of the dimples can range from 0.1 inches to 0.25 inches, with 0.1618 inches being the preferred embodiment. In addition, the vertical portion of the dimple transitions to the flat base with a large radius curve ranging from 0.0625 inches to 0.1875 inches, with 0.125 inches being preferred, and transitions to the flat top with a large radius curve ranging from 0 inches to 0.0625 inches with 0.03125 inches being preferred. These large radius curves prevent thinning of the plastic during the molding process and reduce later breakage at the curved areas. The flat tops of the dimples provide solid support of the adjoining plastic plate and prevent leakage of that plate which might result from penetration by a dimple with a sharp point. The average diameter of the base portion of the dimples ranges from 0.110 inches to 0.360 inches with the preferred dimension being 0.250 inches. The average spacing of the axes of the dimples ranges from 0.75 inches to 1.25 inches with 1 inch being preferred. The average height of the dimples and consequently the spacing between the plates ranges from 0.125 inches to 0.375 inches, with 0.25 inches being the preferred embodiment. There are two versions of the plates with dimples offset so that they do not nest into the adjacent plate. The heat exchanger comprises a stack of the dimpled plates with the dimples offset from each other in alternate layers to provide a strong series of chambers for the two air flow streams. The two air flow streams flow at right angles to each other through the stack. Alternating pairs of plates are sealed at their opposite edges to create two sets of flow channels for the air streams through the stack. The plate edges are sealed by a 100% silicone adhesive sealant (see page 17) between them, and aerodynamically shaped long U-shaped clips (containing an additional viscous sealant) are pushed over the sealed plate edges and cover most of their length. The U-shaped clips can be made of plastic similar to the heat exchanger plates or various metals of varying thicknesses, but 50 mils for each side of the U-shape is preferred. There are small gaps at all four corners which are not covered by the U-shaped clips. These gaps are sealed by four L-shaped brackets containing a sealant inside the L-shape. The U-shaped clips provide a rigid and uniform interface for the L-shaped brackets and sealants therein. The L-shaped brackets along all four corners of the stack of plates also hold the assembly of plates in the shape of a rigid cube or rectangular solid. The L-shaped brackets are also rigidly attached to metal end plates the same size as the heat exchanger plates, and the L-shaped brackets are also attached together by one or more metal pressure bands around the brackets and stack of plates. The brackets, pressure bands, and end plates provide structural integrity to the stack. In the preferred embodiment, the plates are prevented from moving with respect to each other by two or more alignment lugs which are perpendicular to the plates and consist of a detente which fits over a dimple in the adjoining plate. The invention also includes the process for making the flat top dimples from thermoplastic sheeting in a manner which minimizes later breakage in the thermoplastic sheet, or in the adjacent thermoplastic sheet at the point of contact by the dimples. The invention also includes the method of assembling and sealing the set of dimpled plates to form a leak-proof rigid structure through which the two air streams can flow in proximity to each other and thereby transfer heat from the primary air being introduced into the space for human comfort or process, to the cooled humid air produced by evaporation of water in the secondary air stream within the structure. The invention can be used with many gaseous fluid streams, not just air streams, and it could be used without evaporating water, in which case the efficiency is less.